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Ann. Geophys., 17, 613-622, 1999
www.ann-geophys.net/17/613/1999/
© European Geosciences Union 1999
Nongyrotropic particle distributions in space plasmas
U. Motschmann1,2, K. H. Glassmeier3, and A. L. Brinca4
1German Aerospace Center (DLR), Institute for Planetary Exploration, D-12484 Berlin, Germany
2Now at Institute for Theoretical Physics, Technical University, D-38106 Braunschweig, Germany
E-mail: U.Motschmann@tu-bs.de
3Institute for Geophysics and Meteorology, Technical University, D-38106 Braunschweig, Germany
4Centro de Fisica de Plasmas, Instituto Superior Tecnico, P-1096 Lisboa Codex, Portugal
Abstract. In nonstationary, strong inhomogeneous or
open plasmas particle orbits are rather complicated. If the nonstationary time
scale is smaller than the gyration period, if the inhomogeneity scale is smaller
than the gyration radius, i.e. at magnetic plasma boundaries, or if the plasma
has sources and sinks in phase space, then nongyrotropic distribution functions
occur. The stability of such plasma configurations is studied in the framework
of linear dispersion theory. In an open plasma nongyrotropy drives unstable
waves parallel and perpendicular to the background magnetic field, whereas in
the gyrotropic limit the plasma is stable. In nonstationary plasmas nongyrotropy
drives perpendicular unstable waves only. Temporal modulation couples a seed
mode with its side lobes and thus it renders unstable wave growth more
difficult. As an example of an inhomogeneous plasma a magnetic halfspace is
discussed. In a layer with thickness of the thermal proton gyroradius a
nongyrotropic distribution is formed which may excite unstable parallel and
perpendicular propagating waves.
Key words. Interplanetary physics (plasma waves and
turbulence) · Ionosphere (plasma waves and instabilities) · Magnetospheric
physics (plasma waves and instabilities)
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